The invention concerns a circuit for electrothermal measurement of level compensated for ambient temperature, which has a switched source of constant current for a resistance probe, a comparator which compares a first variable formed from an initial voltage U.sub.0 at the resistance probe at the start of a heating period with a second variable formed from a heating voltage U.sub.1 at the end of a heating period, the formation of at least one of the two variables taking place in accordance with a time function determined by a capacitor and a resistor.
In one such known circuit, the resistance probe is connected upon the start of each measurement (beginning of a measurement period) to the source of constant current which is actuated during the heating period by a program control (delay circuit). The latter is connected to the comparator. The comparator is connected, on the one hand, with an inverter which is connected to the resistance probe and to a resistor-capacitor member connected in parallel thereto, in such a manner that the comparator gives off an output signal to a monitoring device as soon as the voltage drop over the resistor-capacitor member is equal to that of the inverter (Federal Republic of Germany OS No. 27 40 289). The principle of measurement is based on the measurement of the change in resistance, obtained at the end of a period of heating by a constant current, of the resistance probe which is surrounded to a greater or lesser extent by the fluid to be measured. Ordinarily, the difference between the voltage U.sub.1 at the end of the heating period and an initial voltage U.sub.0 at the beginning thereof is determined. In order for the result of this measurement to be independent of the ambient temperature, the following measures are taken in said prior art: At the time of the connection of the resistance probe, the inverter supplies a voltage V-U.sub.0 which charges the capacitor of the resistor-capacitor member. The latter then discharges in accordance with a known law which is determined by the resistance and the capacitance. When, during the further course of the heating period, the resistance probe heats up, the voltage U increases and the inverter supplies a voltage of the form V-U which is dependent on the depth of immersion of the resistance probe. The time at which the voltage curve of the capacitor (charge curve), which can be represented as a straight line, intersects the curve of the inverted voltage V-U is noted by the comparator. At a time determined by the program control the comparator is controlled in such a manner that a comparison is effected between the value of the inverted voltage V-U.sub.0 at the terminals of the resistor-capacitor member and the value of the voltage present on the inverter. A conitoring device connected to the output of the comparator can, in this connection, give off an alarm when a predeterminable threshold value has been exceeded. In general, according to this prior art, the level of a liquid into which the resistance probe is immersed can also be indicated by digital or analog indicating instruments. This circuit is relatively expensive, in particular because of the inverter which is present. In addition, it cannot be seen how this circuit can be integrated in an analog to digital converter in order to form digital display values.
A circuit similar to this type for the electrothermal measurement of level has also been proposed which has a resistance probe which is heated by a constant current and an evaluation circuit which evaluates a measurement voltage which is dependent on the state of immersion of the resistance probe, into which circuit a combination voltage can furthermore be fed for the temperature compensation, and in which the compensation voltage can be obtained from the resistance probe and stored at the start of the heating period while at the end of the heating period the measurement voltage can be obtained (Patent Application No. DE P 33 37 779.0). For the temperature compensation, the evaluation circuit is adapted to form the quotient of measurement voltage divided by compensation voltage during each sampling period. The measurement voltage and the compensation voltage can be fed in accordance with this principle also into an analog-digital converter to whose output digital storages and a digital arithmetic circuit for the forming of the quotient are connected. This circuit is relatively expensive because of the means for the forming of the quotient. This is true in the case that the evaluation circuit is developed in analog technique for the forming of the quotient and, in particular, when it is developed in digital technique, since here the analog-digital converter must, first of all, convert the measurement voltage and the compensation voltage before their digital values can be further evaluated.
It is an object of the present invention to further develop a circuit of the aforementioned type in such a manner that, while being of simple construction and high precision, it is suitable to provide digital level values and, in addition, can be integrated particularly easily into a corresponding converter.